The present invention relates to a method for operating a storage catalytic converter of an internal combustion engine in particular of a motor vehicle, in which the storage catalytic converter is filled with and emptied of nitrogen oxide. The present invention also relates to a control unit for an internal combustion engine, in particular a motor vehicle, as well as to an internal combustion engine, in particular for a motor vehicle.
A method of this type, a control unit of this type, and an internal combustion engine of this type are familiar, for example, in a so-called direct fuel-injection system. In that context, the fuel is injected into the combustion chamber of the internal combustion engine, in homogeneous operation, during the intake phase or, in stratified operation, during the compression phase. Homogeneous operation is advantageously provided for the full-load operation of the internal combustion engine, whereas stratified operation is suited for idling and partial-load operation. By way of example, in a direct-injection internal combustion engine of this type, the switchover between the aforementioned operating modes is carried out as a function of the requested torque.
In particular, in order to carry out stratified operation, a storage catalytic converter is used to temporarily store the nitrogen oxides that are generated, so as to reduce them in a three-way catalytic converter during a succeeding homogeneous operation. This storage catalytic converter is filled with nitrogen oxides in stratified operation and is emptied once again in homogeneous operation. This filling and emptying leads to an aging of the storage catalytic converter and therefore to a diminution of its storage capacity.
It is the objective of the present invention to create a method for operating a storage catalytic converter of an internal combustion engine, which makes it possible to detect the aging of the storage catalytic converter.
This objective is achieved in a method of the aforementioned type in accordance with the present invention, in that a plurality of aging conditions of the storage catalytic converter are determined, and in that, from the aging conditions, a reversible aging curve and a nonreversible aging curve are determined. In a control unit and an internal combustion engine of the aforementioned types, the objective is achieved accordingly.
The reversible aging curve represents a decrease, caused by sulfur, in the storage capacity of the storage catalytic converter. The sulfur can be contained, for example, in the fuel or at least in the exhaust gas arising in the internal combustion engine. The non-reversible aging curve represents a natural decrease, caused by use, in the storage capacity of the storage catalytic converter. This decrease occurs in particular as a result of the thermal stress on the catalytic converter after an appropriately long service life.
By determining the reversible and the non-reversible aging curves, the distinction can be precisely made by the control unit, as to whether a regeneration of the storage catalytic converter is possible, or whether the storage catalytic converter is actually used up. As a result, it can be avoided that a storage catalytic converter that can actually be regenerated is recognized as defective and, for example, is replaced.
In one advantageous refinement of the present invention, the reversible aging curve is determined from the sequential aging conditions between two regenerations of the storage catalytic converter. In addition, the non-reversible aging curve is determined from the aging conditions after each regeneration of the storage catalytic converter. Therefore, it is possible in a simple manner to distinguish the different aging curves from each other.
It is especially advantageous if, for distinguishing the reversible and the non-reversible aging curves, the slope of the reversible aging curve is compared to the slope of the non-reversible aging curve. In this context, the slope of the reversible aging curve is greater than the slope of the non-reversible aging curve. In this way, a reliable differentiation of the two aging curves is assured.
In one advantageous embodiment of the present invention, the storage catalytic converter is regenerated after a specifiable reversible aging condition has been reached. Thus it is assured that the storage catalytic converter will not be replaced on account of a reversible aging, but it will be regenerated instead.
In a further advantageous embodiment of the present invention, the storage catalytic converter is recognized as defective when a specifiable non-reversible aging condition has been attained. Therefore, the storage catalytic converter is only recognized as defective when it can no longer, or only slightly, be regenerated. The replacement of the storage catalytic converter is therefore avoided until the non-reversible aging curve reaches a specifiable threshold value.
Of particular importance is the realization of the method according to the present invention in the form of a control element, which is provided for a control unit of an internal combustion engine, in particular of a motor vehicle. In this context, a program that can be executed on a computing device, in particular on a microprocessor and that is suitable for carrying out the method according to the present invention is stored on the control element. In this case, the present invention is realized using a program that is stored on the control element, so that this control element, which is provided with a program, represents the present invention in the same way as the method which the program is designed to carry out. As the control element, an electrical storage medium can especially be used, for example, a read-only memory or a flash memory.